In fluid bed combustors, a stream of gas flows upward through a dense bed of solid fuel particles with sufficient force to suspend and tumble these particles together, thereby giving the bed the appearance of a boiling fluid. These suspended solid particles, which often consist of a combination of coal and limestone, are burned so as to generate heat which is absorbed through adjacent tubing. The sulfur dioxide (SO.sub.2) exhaust from the coal is captured before its release into the atmosphere by its reaction with the calcium oxide (CaO) given off by the limestone thereby forming calcium sulfate (CaSO.sub.4), a dry solid. A substantial amount of this chemical reaction or sulfur capture occurs while these exhaust gases mix in the freeboard area of the combustor above the fluid bed, and the completion of this freeboard reaction is directly related to the residence time of the gases in the combustor. If the exhaust gases are vented too quickly, or are vented without sufficient mixing, the amount of sulfur dioxide captured will be significantly reduced which generally results in unacceptably high levels of sulfur discharge.
Currently, the gas retention time within fluid bed combustors is increased by stacking fluid beds either vertically as shown in U.S. Pat. Nos. 3,905,336 and 4,135,885, or side-by-side or horizontally as shown in U.S. Pat. No. 3,893,426. These combustors are all of the atmospheric combustion type because combustion takes place at or near atmospheric pressure. In contrast, a pressurized fluidized bed combustor is shown in U.S. Pat. No. 3,863,606 in which combustion occurs at a pressure of several atmospheres.
Generally, stacked fluid beds are constructed in a parallel arrangement such that the flue gases from one bed do not flow around another bed but instead are separately channeled to the convection pass adjacent the combustion chamber and furnace shaft before being exhausted. This reduces the completion of the chemical reaction by reducing the gas retention time within the combustor. Thus, in existing fluid bed designes, thorough mixing of the gases in the freeboard area has been challenged because complete mixing of the flue gases from all the beds does not occur until these gases have reached the convection pass by which time the temperature of these gases has decreased below the optimum reaction temperature window which inhibits further reactions.
Additionally, the amount of limestone required for adequate sulfur capture, identified as the Ca/S ratio, is dependent upon the fuel sulphur content and the degree of limestone utilization achieved. Ideally, for a low, efficient Ca/S ratio, the calcined limestone in each of the beds should be consumed as completely as possible with any remaining unreacted lime being recycled for further consumption. A desirable way to decrease the Ca/S ratio is to increase the freeboard gas retention time which will thereby improve the overbed combustion efficiency and increase calcium utilization.
It is thus an object of this invention to provide an improved fluid bed combustor having a prolonged flue gas path thereby increasing the gas retention time in the combustor. Another object of this invention is to provide a combustor which increases the mixing of the flue gases and which promotes the completion of the sulfur capturing reaction by having a cumulative serpentine flue gas path within the combustor itself. It is a further object of this invention to provide a system of recycling spent fuel which enables the complete combustion of the fuel to occur thereby lowering the Ca/S ratio. These and other object and advantages of this invention are described in detail as follows: